Epoxy resin matrices are usually two-component systems, comprising a resin component [e.g. a diglycidyl ether of bisphenol A (DGEBA)] and a crosslinking agent [e.g. p,p'diaminodiphenylmethane (DDM)]. Diluents or fillers are also often added for economic or processing reasons. The resin and curing agent are mixed and "cured", usually at elevated temperatures. The above matrix would typically have a strength of 85 MPa, a modulus of 2800 MPa, a glass transition temperature (T.sub.g, a maximum use temperature) of .about.150.degree. C. and would fracture in an essentially brittle fashion (H. Lee and K. Neville, "Handbook of Epoxy Resins", McGraw-Hill, New York, 1967). Considerable effort has been expended to improve on these properties.
However, no completely satisfactory solution currently exists because improvement in one property has been gained only at the expense of another (e.g. solvent resistance, T.sub.g, ductility, processability, cost).
Reaction products of aromatic polyhydroxy compounds and epoxides (particularly epihalohydrins) are widely used in epoxy technology, since the reaction is one step in the synthesis of many epoxy resins. A typical synthesis may involve reacting 1 mole of 4,4'-isopropylidene bisphenol (bisphenol A) with 10 moles of epichlorohydrin at about 115.degree. C., while slowly adding 2 moles of 40% aqueous sodium hydroxide. Excess epichlorohydrin is distilled away and the crude product is purified by washing and filtration to give a resin forming polyepoxide commonly referred to as the diglycidyl ether of bisphenol A (DGEBA). Details of this and other similar syntheses may be found in "Handbook of Epoxy Resins" by H. Lee and K. Neville, McGraw-Hill, New York, 1967.
Antiplasticization in highly crosslinked epoxy resin systems has been reported in the literature as a means of modifying their properties. For instance, N. Hata et al, in J. Appl. Polym. Sci., 17(7), p. 2173-81, 1973, describe pentachlorobiphenyl as the most effective antiplasticizer, and dibutylphthalate and 2,2-bis[4-2-(2-hydroxy-3-phenoxypropoxy)phenyl]propane (a reaction product of DGEBA and phenol) as less effective antiplasticizers. There evidently has been little recent interest in this, possibly because materials such as pentachlorobiphenyl are environmentally objectionable.
We have found an alternative method of improving the properties of epoxy resins, by adding reaction products which act as fortifiers, to conventional epoxy resin+curing agent formulations.